A/D converter for wideband digital communication

ABSTRACT

Circuitry for providing non-uniform analog-to-digital (“A/D”) signal conversion for wideband signals is provided. The circuitry of the invention is optimized for wideband signals because it does not sacrifice the small-scale resolution of high-probability signal amplitudes while preventing the clipping of low-probability signal amplitudes. The circuitry includes a nonlinear amplifier and an A/D converter that may be uniform or non-uniform. The digital output of the A/D converter may be further processed by circuitry that has an output function that is the inverse of that of the nonlinear amplifier, so as to maintain linear A/D conversion.

This application claims the benefit of U.S. Provisional Application No.60/592,291, filed Jul. 28, 2004, which is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to analog-to-digital (“A/D”) signal conversion inwideband digital communication. Specifically, the invention relates toimproving the dynamic range of A/D converters (“ADCs”).

Currently, in many areas of high-speed digital communication, analogsignals are processed and converted into digital data by beingdownconverted, filtered, and amplified to a particular analog basebandfrequency prior to A/D conversion. As a result, such signals areconfined to certain frequency boundaries and therefore make ADCs withuniform quantization (i.e., equal granularity or step sizes betweenincrements) an acceptable solution for such applications.

However, the recent advent of digital cable television and cellularphone technologies such as CDMA (Carrier Division Multiple Access) hasbrought about an increase in the use of wideband digital signals. Thesesignals, which may typically be approximated by a Gaussian probabilitydistribution function, are generally small in amplitude with extremelylarge amplitude spikes. As a result, the dynamic range of the previousuniform ADCs are no longer adequate, since they would result in eitherthe sacrificing of small-scale resolution or else would clip thelarge-amplitude signals due to their limited range.

It would therefore be desirable to design an ADC with non-uniformgranularity for wideband signals that require a large dynamic range. Itwould further be desirable to design an ADC with non-uniformgranularity, without sacrificing small-scale resolution or clipping thelarge amplitude signal components. Finally, it would be desirable todesign an ADC with non-uniform granularity and without a costly increasein the amount of hardware required to implement the ADC.

SUMMARY OF THE INVENTION

According to the present invention, circuitry for performing non-uniformanalog-to-digital signal conversion for accommodating wideband signalssuch as those previously described is provided. The circuitry may beimplemented using a linear ADC. The ADC is preceded by a nonlinearamplifier that may demonstrate, for example, a relatively larger gainfor input signals with comparatively small amplitudes and a relativelysmaller gain for input signals having comparatively larger amplitudes.The combined effect of the nonlinear amplifier and the ADC is to provideanalog-to-digital conversion that exhibits small quantization noise forsmall-amplitude (i.e., high probability) signals and larger quantizationnoise for large-amplitude (i.e., low probability) signals, for instance.The resultant digital signals from the ADC may be subsequently processedby inverse transfer function circuitry in order to preserve a linearrelationship between the input and output signals.

The circuitry of the present invention demonstrates increasedsmall-scale resolution without clipping the large-scale signalcomponents or requiring substantial additional circuitry. Theimprovement in the signal-to-noise (“SNR”) ratio of the non-uniform ADCcompared to a uniform ADC for these wideband signal inputs issubstantial. For example, for an input consisting of 10 QAM (QuadratureAmplitude Modulation) channels in which one channel has 10 dB less powerthan the other channels (typical of cable television and wirelesscommunication), the SNR of a 7-bit ADC that is preceded by a nonlinearamplifier implementing a square-root function may be 10 dB better thanthat of a normal uniform 7-bit ADC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an illustrative embodiment ofA/D conversion circuitry in accordance with the present invention; and

FIG. 2 is an illustrative circuit diagram of a nonlinear amplifierportion of the A/D conversion circuitry in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an illustrative block diagram of the circuitry of thepresent invention. The circuitry in FIG. 1 includes a first nonlinearamplifier 10, ADC 12, and inverse transfer function circuitry 14. Analoginput signals are first transmitted to nonlinear amplifier 10. Nonlinearamplifier 10 amplifies the input signals according to a nonlinear outputfunction f(x). The amplified signals are then passed on to ADC 12. ADC12 digitizes the amplified incoming analog signals, and subsequentlyoutputs the converted digital signals to inverse transfer functioncircuitry 14. Inverse transfer function circuitry 14 has an outputfunction that is the inverse (i.e., f⁻¹(x)) of that of nonlinearamplifier 10. Circuitry 14 performs a nonlinear mapping to the receiveddigital signals that results in the desired linear relationship betweenthe analog input and the final digital output signals.

FIG. 2 is an illustrative circuit diagram of nonlinear amplifier 10 inaccordance with the present invention. In particular, nonlinearamplifier 20 in FIG. 2 includes amplifier 22, resistors 24 and 26, andmatched diodes 28 and 30. Amplifier 22 may be for instance anoperational amplifier (“op-amp”). Diodes 28 and 30 are connected inparallel facing opposite directions and are matched to each other so asto cause nonlinear amplifier 20 to produce an output that has zero-inputsymmetry. In other words, the output of nonlinear amplifier 20 isdependent only on the magnitude of the input.

The gain of nonlinear amplifier 20 is determined by the electricalcharacteristics of resistors 24 and 26 and of diodes 28 and 30.Specifically, the amount of current I across diodes 28 and 30 for agiven voltage drop V across the diodes is (the same equation applies toboth diodes):I=I _(o)×(exp(V/V _(o))−1)where I_(o) and V_(o) are constant values that depend on thecharacteristic of each corresponding diode. Plugging this expressioninto the equation for the impedance Zd for each diode yields:Z _(d) =V/I=V/(I _(o)×((exp(V/V _(o))−1)

The diode impedance Z_(d,28) for diode 28—the diode with the positiveconductivity in the circuit arrangement shown in FIG. 2—may subsequentlybe used in determining the output function of nonlinear amplifier 20(the contribution of diode 30 to the output function is negligiblecompared to that of diode 28 and therefore may be ignored).Specifically, the output function of nonlinear amplifier 20 expressed interms of the R₁ (the resistance of resistor 24), R_(F) (the resistanceof feedback resistor 26) and Z_(d,28) is given by the equation:f(x)=x(t)×(R _(f) ×Z _(d,28)/(R _(f) +Z _(d,28)))/R ₁

It is thus seen from the above expressions that when the input voltagesignal is small (i.e., for small amplitude signals), the impedance ofdiode 28 is much larger than Rf, and as a result the gain of nonlinearamplifier 20 is approximately Rf/R1. On the other hand, when the inputsignal is large, the impedance of diode 28 is much smaller than Rf, andas a result the gain of nonlinear amplifier 20 is logarithmicallyrelated to the input signal. Therefore, the overall effect of nonlinearamplifier 20 is to limit the amplifier gain for small-amplitude inputsignals and to compress the amplifier gain for large-amplitude inputsignals. As a result, when the amplified signals are transmitted to theADC and quantized, the quantization step that is applied to thesmall-amplitude input signals is effectively smaller than thequantization step applied to the large-amplitude input signals, therebyallowing a uniform ADC to achieve non-uniform granularity at littleadditional cost in hardware. The non-uniform granularity created by thenonlinear amplifier helps to reduce the quantization noise created by afixed-resolution ADC and substantially improves the SNR of the ADC.

After the analog input signals have been processed by the nonlinearamplifier 10, the nonlinear data is digitized by ADC 12 and subsequentlysent to inverse transfer function circuitry 14. The purpose of inversetransfer function circuitry 14 is to remap the nonlinear digital data sothat the final digital output of the ADC circuitry is a linear functionof the analog input. Therefore, if the nonlinear amplifier 14implemented the output function shown above with respect to FIG. 2,inverse transfer function circuitry 14 would implement an outputfunction given by the following inverse expression:f ⁻¹(x)=x(t)×((R _(f) +Z _(d))/R _(f) ×Z _(d)) R₁Although an exemplary circuit implementation exhibiting this particularoutput function is not shown, different circuit implementations for sucha function will be readily apparent to one of ordinary skill in the art.For example, similar to nonlinear amplifier 10, inverse transferfunction circuitry 14 may also utilize an op-amp along with matchingdiodes and linear resistors.

It will be understood from the foregoing that although the presentinvention has been described herein with respect to a particular type ofnonlinear amplifier and ADC, other variations of nonlinear amplifiersand ADCs may be used without departing from the scope of the invention.For example, alternative nonlinear amplifiers that implement differentoutput functions or use different circuit components may be employed.The specific output functions of the nonlinear amplifier—which therebyaffects the granularity of the ADC—and of the corresponding inversetransfer function circuitry may be altered according to the probabilitydensity functions of the particular input signals of a givenapplication. Furthermore, other types of ADCs besides uniform, linearADCs may be used. For example, different types of nonlinear ADCs (e.g.,ADCs with a stepwise output function) may be used in accordance with thepresent invention in order to further improve the dynamic range of thecircuitry. Alternatively, a nonlinear ADC may be realized by not usinginverse transfer function circuitry following a linear ADC. Depending onthe particular application, the ADC may be implemented using a flashADC, a sigma-delta ADC, or any other type of ADC that is known to one ofordinary skill in the art.

It will be understood, therefore, that the foregoing is onlyillustrative of the principles of the invention, and that variousmodifications can be made by those skilled in the art without departingfrom the scope and spirit of the invention, and that the presentinvention is limited only by the claims that follow.

1. Circuitry for performing analog-to-digital (A/D) conversion, thecircuitry comprising: a nonlinear amplifier, wherein the nonlinearamplifier has an output function that applies a relatively higher gainto analog input signals having higher-probability signal amplitudes anda relatively lower gain to the analog input signals havinglower-probability signal amplitudes, and further wherein the nonlinearamplifier comprises; an amplifier having a first input and a secondinput and an output; a first resistor having a first and a secondterminal, wherein the first terminal is operative to receive the inputsignals and the second terminal is in communication with the first inputof the amplifier; a second resistor in communication with the firstinput and the output of the amplifier; a first diode having an input andan output, wherein the input is in communication with the first input ofthe amplifier and the output is in communication with the output of theamplifier; and a second diode having an input and an output, wherein theinput is in communication with the output of the amplifier and theoutput is in communication with the first input of the amplifier; and anA/D converter in communication with the nonlinear amplifier.
 2. Thecircuitry defined in claim 1 wherein the A/D converter is a uniform A/Dconverter.
 3. The circuitry defined in claim 1 wherein the A/D converteris a non-uniform A/D converter.
 4. The circuitry defined in claim 1wherein the A/D converter is a linear A/D converter.
 5. The circuitrydefined in claim 1 wherein the A/D converter is a nonlinear A/Dconverter.
 6. The circuitry defined in claim 1 wherein the A/D converteris a flash A/D converter.
 7. The circuitry defined in claim 1 whereinthe A/D converter is a sigma-delta A/D converter.
 8. The circuitrydefined in claim 1 wherein the higher-probability signal amplitudescorrespond to the analog input signals having relatively smalleramplitudes and the lower-probability signal amplitudes correspond to theanalog input signals having relatively larger amplitudes.
 9. Thecircuitry defined in claim 1 further comprising: inverse transferfunction circuitry in communication with the A/D converter, wherein theinverse transfer function circuitry has art output function that is theinverse of the output function of the nonlinear amplifier.
 10. Circuitryfor performing analog-to-digital (A/D) conversion, the circuitrycomprising: a nonlinear amplifier having a first output function,wherein the nonlinear amplifier comprises: an amplifier having a firstinput and a second input and an output; a first resistor having a firstand a second terminal, wherein the first terminal is operative toreceive the input signals and the second terminal is in communicationwith the first input of the amplifier; a second resistor incommunication with the first input and the output of the amplifier; afirst diode having an input and an output, wherein the input is incommunication with the first input of the amplifier and the output is incommunication with the output of the amplifier; and a second diodehaving an input and an output, wherein the input is in communicationwith the output of the amplifier and the output is in communication withthe first input of the amplifier; an A/D converter in communication withthe nonlinear amplifier; and inverse transfer function circuitry incommunication with the A/D converter, wherein the inverse transferfunction circuitry has a second output function that is the inverse ofthe first output function of the nonlinear amplifier.
 11. The circuitrydefined in claim 10 wherein the A/D converter is a uniform A/Dconverter.
 12. The circuitry defined in claim 10 wherein the A/Dconverter is a non-uniform A/D converter.
 13. The circuitry defined inclaim 10 wherein the A/D converter is a linear A/D converter.
 14. Thecircuitry defined in claim 10 wherein the A/D converter is a nonlinearA/D converter.
 15. The circuitry defined in claim 10 wherein the A/Dconverter is a flash A/D converter.
 16. The circuitry defined in claim10 wherein the A/D converter is a sigma-delta A/D converter.
 17. Thecircuitry defined in claim 10 wherein the first output function of thenonlinear amplifier applies a relatively higher gain to analog inputsignals having higher-probability signal amplitudes and a relativelylower gain to the analog input signals having lower-probability signalamplitudes.
 18. The circuitry defined in claim 10 wherein thehigher-probability signal amplitudes correspond to the analog inputsignals having relatively smaller amplitudes and the lower-probabilitysignal amplitudes correspond to the analog input signals havingrelatively larger amplitudes.
 19. Circuitry for performinganalog-to-digital (A/D) conversion, the circuitry comprising; means foramplifying analog input signals based on an output function that appliesa relatively higher gain to the analog input signals havinghigher-probability signal amplitudes and a relatively lower gain to theanalog input signals having lower-probability signal amplitudes, whereinthe means for amplifying comprises: an amplifier having a first inputand a second input and an output; a first resistor having a first and asecond terminal, wherein the first terminal is operative to receive theinput signals and the second terminal is in communication with the firstinput of the amplifier; a second resistor in communication with thefirst input and the output of the amplifier; a first diode having aninput and an output, wherein the input is in communication with thefirst input of the amplifier and the output is in communication with theoutput of the amplifier; and a second diode having an input and anoutput, wherein the input is in communication with the output of theamplifier and the output is in communication with the first input of theamplifier; and means for converting the amplified analog input signalsto digital signals.
 20. The circuitry defined in claim 19 wherein thehigher-probability signal amplitudes correspond to the analog inputsignals having relatively smaller amplitudes and the lower-probabilitysignal amplitudes correspond to the analog input signals havingrelatively larger amplitudes.
 21. The circuitry defined in claim 19further comprising: means for applying to the digital signals an outputfunction that is the inverse of the output function of the means foramplifying the analog input signals.
 22. Circuitry for performinganalog-to-digital (A/D) conversion, the circuitry comprising: means foramplifying analog input signals based on a first output function,wherein the means for amplifying comprises: an amplifier having a firstinput and a second input and an output; a first resistor having a firstand a second terminal, wherein the first terminal is operative toreceive the input signals and the second terminal is in communicationwith the first input of the amplifier; a second resistor incommunication with the first input and the output of the amplifier; afirst diode having an input and an output, wherein the input is incommunication with the first input of the amplifier and the output is incommunication with the output of the amplifier; and a second diodehaving an input and an output, wherein the input is in communicationwith the output of the amplifier and the output is in communication withthe first input of the amplifier; means for converting the amplifiedanalog input signals to digital signals; and means for applying to thedigital signals a second output function that is the inverse of thefirst output function.
 23. The circuitry defined in claim 22 wherein thefirst output function applies a relatively higher gain to the analoginput signals having higher-probability signal amplitudes and arelatively lower gain to the analog input signals havinglower-probability signal amplitudes.
 24. The circuitry defined in claim23 wherein the higher-probability signal amplitudes correspond to theanalog input signals having relatively smaller amplitudes and thelower-probability signal amplitudes correspond to the analog inputsignals having relatively larger amplitudes.